Allow failures for HHVM

Author: cerbero90

.travis.yml

@@ -20,6 +20,8 @@ matrix:
   include:
     - php: 7.1
       env: 'COMPOSER_FLAGS="--prefer-stable --prefer-lowest"'
+    allow_failures:
+        - php: hhvm
 
 before_script:
   - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --prefer-dist